Implement DNS spoofing for SRV records (working)

The current version of the dns_spoof plugin contains just some incomplete fragments of code to spoof SRV records which actually didn't work.

I rewrote the code to enable dns_spoof plugin spoofing SRV records of course for IPv4 and IPv6 addresses. In addition I restructured the comments and examples for SRV spoofing in the file etter.dns.

Additionally I updated the dns_spoof plugin documentation (man page).